In prior art, an elevator system comprises an elevator car which is connected to a counterweight via suspension ropes which go over a traction wheel driven by a hoisting machine. The elevator car and the counterweight are both guided vertically by respective guide rails inside a shaft. In the following, the elevator car and the counterweight are referred to as the moving mass. The elevator system further comprises a safety circuit having a plurality of normally closed safety switches for monitoring the safety status of the elevator in normal operation. If the safety of the elevator is somehow compromised, at least one of the safety switches is opened, the hoisting machine is deenergized and machinery brakes are engaged so as to decelerate the moving mass for quick stop.
The elevator system further comprises an overspeed governor system for the elevator car, which has a governor rope loop directed up from the elevator car, over an overspeed governor pulley, then down and under a tension weight pulley connected to a tension weight and then up again to the elevator car to be connected to a synchronization linkage for tripping an elevator car safety gear. A corresponding overspeed governor system can be attached to the counterweight.
The synchronization linkage has synchronization levers which make the safety gear of the moving mass to engage the guide rails of the moving mass when at least a predetermined force is applied to the synchronization linkage by the governor rope. This predetermined force is acting against spring forces of synchronization lever springs such that the synchronization lever engages the safety gear when the force applied by the governor rope exceeds the synchronization lever spring force. The overspeed governor system supervises the speed of the moving mass, and, if this speed exceeds a predetermined tripping speed which is above a rated speed of the elevator, it activates the machinery quick stop operation and, simultaneously, decelerates the governor rope. This deceleration of the governor rope acts against the spring forces of synchronization lever springs such that the synchronization lever engages the safety gear, bringing the elevator car into an emergency stop.
To summarize, a quick stop operation of the machinery is initiated whenever the elevator safety circuit indicates a compromised safety status of the elevator. Additionally, if the compromised safety status is a result of an overspeed condition of the moving mass, detected by overspeed governor, an emergency stop operation is activated by engaging the safety gear of the moving mass.
However, in high rise elevators, the elevator travel and speed increase such that the inertia of the governor rope increases substantially. This brings a new challenge during elevator quick stops carried out by the hoisting machine brakes. Namely, when the overspeed governor rope having the increased length decelerate during the above explained quick stop, a large force is applied to the synchronization linkage, because the inertia of the overspeed governor rope is large. As a result, the decelerating governor rope is capable of producing forces to the synchronization linkage which exceed the needed force to engage the safety gear when the moving mass is decelerated. In other words, the safety gear might be unwantedly engaged or tripped during quick stop although the speed of the moving mass has not exceeded the predetermined tripping speed for engaging the safety gear.
One solution for preventing unwanted safety gear tripping is to increase the synchronization lever spring force. However, this has an effect on the design of the overspeed governor since the European lift standard EN-81-20 code requires that the tensile force in the overspeed governor rope produced by the governor, when tripped, shall be twice the force that is necessary to engage the safety gear via the synchronization linkage. Stronger synchronization leads to bigger overspeed governor rope tensile forces and, consequently a stronger and, thus, heavier overspeed governor rope due to required safety factor. If one wishes to increase the force required for tripping the safety gear by increasing the synchronization lever spring force to oppose the inertial force of the governor rope, then, due to the EN-81-20 code requirement, the tensile strength of the governor rope would have to be increased which would cause the need for redesigning of the overspeed governor system. It is evident that this will finally lead to elevator systems in which there is no more feasible design window for overspeed governor and safety gear system.
Prior art systems, as known from e.g. documents JP 2626408, U.S. Pat. Nos. 7,128,189, 7,475,756 utilize springs and U.S. Pat. No. 4,083,432 utilizes a spring loaded weight for the same purpose.